Mesoscopic Quantum Electrodynamics with a single spin

International audienceA new type of experiments combining microwave cavities and mesoscopic circuits gathering nanoconductors and fermionic reservoirs has recently appeared [1,2,3]. This mesoscopic Quantum Electrodynamics (QED) offers many new possibilities like for instance quantum computing schemes based on localized electronic spins, or a powerful photonic study of electronic transport.In the first part of this seminar, I will introduce a general theoretical framework to describe these experiments. This task faces two challenges. First, one has to quantize the electromagnetic field properly by taking into account electromagnetic boundary conditions which are naturally omitted in atomic cavity QED, due to the smallness of an atom. Second, in the nanocircuits, one has to take into account collective plasmonic modes, as well as electronic quasiparticle states which are absent from circuit QED performed with superconducting quantum bits. I will present a description of mesoscopic QED experiments which takes into account these specificities [4].In the second part of this seminar, I will present experimental results demonstrating the coherent coupling of a single spin to photons stored in a microwave resonator. Using a circuit design based on a nanoscale spin-valve [5], we coherently hybridize the individual spin and charge states of a double quantum dot while preserving spin coherence. This scheme allows us to increase by five orders of magnitude the natural (magnetic) spin-photon coupling, up to the MHz range at the single spin level. Our coupling strength yields a cooperativity which reaches 2.3, with a spin coherence time of about 60ns [6]. We thereby demonstrate a mesoscopic device which could be used for non-destructive single spin read-out and distant spin/spin coupling via virtual cavity photons. [1] M. R. Delbecq, V. Schmitt, F. D. Parmentier, N. Roch, J. J. Viennot, G. Fève, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. 107, 256804 (2011). [2] T. Frey, P. J. Leek, M. Beck, A. Blais, T. Ihn, K. Ensslin, & A. Wallraff, Phys. Rev. Lett. 108 046807 (2012). [3] K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck & J. R. Petta, Nature 490, 380 (2012) [4] A. Cottet, T. Kontos & B. Douçot, arXiv:1501.00803 [5] A. Cottet & T. Kontos, Phys. Rev. Lett. 105, 160502 (2010). [6] J.J. Viennot, M.C. Dartiailh, A. Cottet & T. Kontos, submitte